System.DmlException: Insert failed. First exception on row 0; first error: MIXED_DML_OPERATION, 非設定オブジェクトを更新した後の設定オブジェクト上の DML 操作 (またはその逆) は、許可されていません: Account、元のオブジェクト: User: [][Salesforce]
対策案
・対応前
@isTest
private class TestRunAs {
   public static testMethod void testRunAs() {
        // Setup test data
        // Create a unique UserName
        String uniqueUserName = 'standarduser' + DateTime.now().getTime() + '@testorg.com';
        // This code runs as the system user
        Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
        User u = new User(Alias = 'standt', Email='standarduser@testorg.com',
        EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US',
        LocaleSidKey='en_US', ProfileId = p.Id,
        TimeZoneSidKey='America/Los_Angeles',
         UserName=uniqueUserName);
        System.runAs(u) {
              // The following code runs as user 'u'
              System.debug('Current User: ' + UserInfo.getUserName());
              System.debug('Current Profile: ' + UserInfo.getProfileId());
          }
    }
}対応後
@isTest
private class TestRunAs2 {
   public static testMethod void test2() {
      Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
      User u2 = new User(Alias = 'newUser', Email='newuser@testorg.com',
         EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US',
         LocaleSidKey='en_US', ProfileId = p.Id,
         TimeZoneSidKey='America/Los_Angeles', UserName='newuser@testorg.com');
      System.runAs(u2) {
         // The following code runs as user u2.
         System.debug('Current User: ' + UserInfo.getUserName());
         System.debug('Current Profile: ' + UserInfo.getProfileId());
         // The following code runs as user u3.
         User u3 = [SELECT Id FROM User WHERE UserName='newuser@testorg.com'];
         System.runAs(u3) {
            System.debug('Current User: ' + UserInfo.getUserName());
            System.debug('Current Profile: ' + UserInfo.getProfileId());
         }
         // Any additional code here would run as user u2.
      }
   }
}